Stokvis‐Reaktion, XXI. Das Verhalten von 9‐Brom‐11H‐dipyrromethen‐1 (10H)‐onen in Essigsäure und über 5‐(1‐Oxo‐1, 10‐dihydro‐11H‐dipyrromethen‐9‐yl)‐1 H‐dipyrromethen‐1,9(11 H)‐dione

Abstract

Durch Erhitzen von 2 in Eisessig bilden sich fünf kristalline Substanzen. Es entstehen neben dem entsprechenden 4,10‐Dihydro‐1 H‐dipyrromethen‐1,9(11 H)‐dion (Wasserstoff‐Propentdyopent) gemäß Schema 1 3, nach Schema 2 5 und 6 sowie gemäß Schema 3 8, von dem die beiden Homologen 9 und 10 beschrieben werden.

Synthesis of Bilins

“…70 When the reaction is carried out in the presence of an aunsubstituted l(lOH)-dipyrrinone (e.g. 52), the synthesis of unsymmetrically substituted biliverdins (e.g. 53) is also possible.i" However, oxidative self-coupling of 9-methyl-l (1OH)-dipyrrinones proved unsuccessful when electron withdrawing substituents (e.g.…”

“…47) either by air in acetic acid solution 62,63 (ef Ref. 52) or by quinones of low redox potential like dichloro-5,6-dicyanobenzoquinone (DDQ)64-67 or chloranil,68,69 in the presence of acid. Thus, mesobiliverdin XIIIa dimethyl ester (48) has been obtained in 85% yield by oxidation of methyl 5'-methylxanthobilirubinate (47) with DDQ in tetrahydrofuran solution.…”

“…66,72 Aside from the straighforward synthetic pathways to biliverdin derivatives mentioned above, the most versatile total syntheses of bile pigments follow a convergent strategy in which l(lOH)-dipyrrinone derivatives (57) play the central role. The latter are usually prepared by base catalyzed condensation of a 3-pyrrolin-2-one (54) 52 with a 5-formylpyrrolecarboxylate (55) as depicted in Scheme 4. In some instances, however, the strong alkaline conditions required for this aldol-like condensation are incompatible with sensitive functionality on the reactants.…”